1. Field of Invention
The presently claimed and disclosed inventive concept(s) relates generally to catalysts and methods of making catalysts and, more particularly, but not by way of limitation, to catalysts and methods of making catalysts of precious metal nanoparticles supported on metal oxide particles.
2. Background of the Invention
Precious metal nanoparticles supported on metal oxides can provide active catalysts for a number of reactions of both environmental and industrial importance. For example, Rodriguez, et al. [1] teach that gold nanoparticles deposited on titania are active catalysts for the low-temperature oxidation of carbon monoxide, the selective oxidation of propene, and photocatalytic oxidations used for environmental cleanup.
Supported precious metal catalysts are typically prepared by one of the following process types: a) co-precipitation, b) deposition/precipitation, or c) impregnation processes. Details of these technologies are described in “Preparation of Solid Catalysts” edited by G. Ertl, et al., Wiley-VCH, 1999; chapter 4, pp. 315-388. In co-precipitation processes, soluble precursors of both the support and the precious metal are precipitated from solution together (e.g., by adjusting the pH) followed by drying, calcination, and reduction of the precious metal precipitate to metallic form. Deposition-precipitation methods involve precipitation (e.g., by adjusting pH) of a salt or hydroxide of the precious metal in the presence of a suspension of the support, followed by drying, calcination and reduction, typically high-temperature gaseous reduction, to form metallic particles. The final preparation process type, impregnation, is achieved by wetting dry support particles with a solution of a solublized precious metal such that the precious metal solution impregnates the pores of the support. Following impregnation, the support is dried, causing the precious metal salt to precipitate in the pores. The support is then calcined and exposed to a reducing gas to form metallic particles within the pores.
Gold was considered to have relatively low catalytic activity until recently, when a Japanese professor Masatake Haruta reported highly active gold catalysts [2,3]. Since then, a large number of papers on supported gold catalysts have been published [4-6]. U.S. Pat. No. 4,698,324 describes a deposition-precipitation method for supported gold catalyst production comprising first immersing a support in an aqueous solution of a gold precursor and a precursor of a base such as urea, aging the mixture at an elevated temperature (e.g., 70° C.), and then separating the solid, drying and calcining. It is claimed that since the base for the gold precipitation is generated in situ by decomposing urea, the precipitated gold hydroxide particles have high homogeneity. Similarly, U.S. Pat. No. 4,839,327, by the same inventors as above, further describes improvement of the deposition-precipitation process for supported gold catalyst production. It is reported that strong binding of ultrafine gold hydroxide particles onto the support has been achieved by precipitating gold species under constantly controlled pH (7-11). Also, U.S. Patent Application Publication No. 2007/0219090 discloses an improved incipient wetness impregnation method which involves first impregnating a porous support with a gold solution such as tetrachloroauric acid and a base solution such as sodium carbonate, then washing the material with water or a base solution to remove chlorine species.
Supported platinum catalysts have been widely used for many years. A common method for platinum catalyst production is impregnation, although a number of other techniques have been reported. U.S. Pat. No. 3,210,296 (1965) discloses production of alumina supported platinum catalyst by impregnating an alumina support with a non-aqueous solution of a platinum compound. The method was said to be advantageous for maintaining the surface area of the support material. U.S. Pat. No. 4,370,260 (1983) discloses a method using a one-step impregnation process to deposit multiple platinum group metals including platinum, palladium and rhodium on metal oxide supports such as alumina. The catalyst was for automobile exhaust treatment. More recently, UK patent application 2 443 895 A (2008) disclosed a platinum catalyst supported on bismuth promoted alumina. The method involves impregnating a dry support material with a solvent containing a catalyst metal. A reducing agent is added to form metal particles in the pores of the support, and the supported metal catalyst is then mixed with a bismuth compound forming bismuth promoted supported metal catalyst.
There has also been significant recent interest and research using silver catalysts in DeNOx applications [7-13]. Silver catalyst preparation is described in U.S. Pat. No. 3,575,888 (1971) disclosing a silver catalyst prepared by aqueous impregnation of porous supports with a solution of reducible silver compound, drying under mild temperature conditions, and treatment of the dried material with a reducing agent in a non-aqueous solvent. U.S. Pat. No. 4,772,578 (1988) discloses a silver deposition method involving first, deposition of a supported metal such as zinc via a vapor phase deposition process, and then treatment of the supported metal material in a solution of a catalyst metal ion species, which deposits on the support via an electrochemical mechanism.
A limitation of the prior art processes is that the particle size and size distribution of the supported precious metal catalyst particles cannot be tightly controlled. Deposition is often within support pores which are not sufficiently exposed to controlled conditions. Also, the deposited species is usually not metallic, and therefore, the particles require secondary treatment to convert to metallic form. Such secondary treatments typically also lower the degree of dispersion and thus, further reduce the effective surface area of the precious metal catalyst particles. The presently claimed and disclosed inventive concept(s) addresses these issues by providing an improved process for making a supported precious metal catalyst.